Two types of known arrangements are generally used at present on board ships to obtain electrical energy of regulated frequency and voltage.
In a first of these arrangements, the propulsion diesel engine rotates at a fixed speed and a conventional alternator is coupled to the engine. The drive speed of the ship is then controlled by a variable pitch propellor. This causes rapid wear of the motor and has relatively low efficiency.
In another of these arrangements, a variable speed propulsion diesel drives a propellor with fixed blades, and the use of a continuously running generating set is then indispensable.
In order to overcome the disadvantages of these arrangements, it has been proposed to use an on board power supply including a first alternator coupled to the diesel engine, an electronic regulator for the first alternator, a rectifier to rectify the current output of the of the first alternator, a direct current motor powered from the output of the rectifier, and a second alternator driven by the direct current motor.
These arrangements are not by themselves entirely satisfactory, since the rectifier sends back upstream to the first alternator, harmonics which cause poor regulation of the alternator. In addition, this regulation is influenced to a certain extent by the accelerations and deceleration of the diesel motor which drives the first alternator. Under these conditions, it is difficult to maintain the output current of the second alternator within an acceptable range of frequency and voltage.
The present invention overcomes these disadvantages.